Hammond Model A, Manual, Page: 111 / 298

TREMULANT SWITCH & CONTROL
MODELS A, AB, BC, C, D, G
The tremulant sometimes called tremolo, is a periodic loudness variation, or change
in intensity. which occurs at a constant frequency. It is fundamentally different
from the vibrato effect, which is created by a periodic raising and lowering of
pitch.
In the Hammond Organ, the tremulant effect is produced and controlled principally by
two components: the tremulant switch and the tremulant control.
The tremulant switch, mounted on the synchronous motor at the extreme left end of
the tone generator, is in effect a variable resistor with no sliding or rubbing
contacts. It consists of an eccentric, geared to the motor shaft, which advances a
laminated bakelite strip so as to alternately make and break 6 contacts in order.
Five resistors are connected to these contacts, ranging in value from 15,000 to
450,000 ohms, together with a length of copper wire of very little resistance. At
one extreme position of the eccentric all contacts are broken and the circuit is
open. At the other extreme all contacts are closed and there is practically no
resistance in the circuit.
The tremulant control, a 130,000 ohm variable resistor mounted on the manual chassis
assembly, is in parallel with the tremulant switch. When this control is turned to a
position of no resistance, the tremulant switch is shorted out. Conversely, when the
control is turned to its maximum resistance, the movement of the eccentric varies
the resistance of the circuit periodically from 0 to 130,000 ohms. This parallel
circuit is in series with the signal from the console, ahead of the pre-amplifier.
Therefore, the signal is varied during each revolution of the eccentric by an amount
depending upon the adjustment of the tremulant control. The tremulant system is not
used in console models having vibrato.
Concert Model E
The tremulant system for Model E organ is the same as that on other models except
that two switches are used. Each switch is mounted on one of the two synchronous
motors that are a part of the main generator and chorus generator respectively, and
each one is connected to one manual. The switch mounted on the main generator
operates at 400 R.P.M. and is connected to the Great manual. The other switch
operates at 348 R.P.M. and is connected to the Swell manual. Two types of tremulant
switches have been supplied, namely, the cage type and the enclosed type. These are
mechanically interchangeable, but replacing the cage type with the enclosed type
does require a slight change in the circuit. In the enclosed type, the condenser
shown as C5 in figure 4 is incorporated within the metal housing. Therefore, the C5
located in the rheostat box is not required and the tremulant switch red wire may be
attached to terminal 6,7, or 8.
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